Electric ducted fan

ABSTRACT

An electric ducted fan for an aircraft is shown. A nacelle defines a duct that houses a propulsive fan having a fan diameter D F . An electric machine is configured to drive the fan, and has an electromagnetically active length L A  and an electromagnetically active diameter D A  defining an aspect ratio (L A /D A ) of from 0.8 to 2. A speed reduction device is located between the electric machine and the fan, and has a reduction ratio of at least 3:1. A ratio of the electromagnetically active diameter D A  to the fan diameter D F  (D A /D F ) is from 0.3 to 0.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromBritish Application No. 1807770.1 filed on May 14, 2018 the entirecontents of which are incorporated by reference.

BACKGROUND Technical Field

This disclosure relates to configurations of electric ducted fans foraircraft.

Description of the Related Art

Ducted fan-type propulsors are used for a large proportion of aircraftdue to their greater efficiency and reduced noise due to lower tiplosses than open propellers. Turbofans utilise a gas turbine core todrive a fan, the fan being larger than the core to produce a bypass flowresponsible for the majority of thrust.

Despite great advances in materials, compressor and turbineaerodynamics, and combustion efficiency, the gas turbine engines used inthe cores of turbofans are still quite thermally inefficient. Inparticular, gas turbines are less efficient the smaller they are, whichrestricts possibilities in terms of the number of engines that may beinstalled on an airframe. Their fuel source is also not renewable.Further, the jet of high enthalpy exhaust from the core is responsiblefor a large amount of noise when it mixes with the exhausted bypassflow.

It is therefore desirable to utilise electric machines in place of gasturbine engines in ducted fan arrangements to alleviate some or all ofthe aforesaid issues.

SUMMARY

The present disclosure is directed towards electric ducted fans foraircraft.

One such electric ducted fan comprises a nacelle defining a duct, and apropulsive fan in the duct having a fan diameter D_(F). An electricmachine is configured to drive the fan via a speed reduction devicehaving a reduction ratio of at least 3:1. The electric machine has anelectromagnetically active length L_(A) and an electromagneticallyactive diameter D_(A) defining an aspect ratio (L_(A)/D_(A)) of from 0.8to 2. A ratio of the electromagnetically active diameter D_(A) to thefan diameter D_(F) (D_(A)/D_(F)) is from 0.3 to 0.5.

Electric ducted fans constructed in such a manner have variousadvantages, such as reduced nacelle drag, a shorter pylon, and increasedground clearance.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 shows an electric ducted fan propulsor which uses an electricmachine to drive a fan via a speed reduction device;

FIG. 2 shows definitions of various geometric parameters;

FIG. 3A shows the propulsor of FIG. 1, using a star-type gearbox as thespeed reduction device;

FIG. 3B shows the propulsor of FIG. 1, using a planetary-type gearbox asthe speed reduction device;

FIG. 4 shows one configuration of the electric machine; and

FIG. 5 shows another configuration of the electric machine.

DETAILED DESCRIPTION

An electric ducted fan propulsor is shown in FIG. 1.

The propulsor is shown generally at 101, attached to a wing 102 of anaircraft (not shown) by a pylon 103.

Being a ducted fan, the propulsor 101 comprises a nacelle 104 whichdefines a duct 105 having an inlet 106 and a nozzle 107, and in which apropulsive fan 108 is located. In operation, the fan 108 raises thepressure of intake air, with swirl in the airflow being removed byoutlet guide vanes 109. The airflow is directed through the nozzle 107to generate thrust.

In the embodiment of FIG. 1, the fan 108 is driven by an electricmachine 110, in combination with a speed reduction device 111.

In the present embodiment, the electric machine 110 is rated at amaximum continuous power of between 100 kilowatts and 100 megawatts. Ina specific embodiment, the electric machine 110 is rated at a maximumcontinuous power of between 1 megawatt and 10 megawatts. In a morespecific embodiment, the electric machine 110 is rated at 2 megawattsmaximum continuous.

In the present example, the speed reduction device 111 is an epicyclicgearbox, but could be another form of reduction device such as alayshaft-based gear system or other suitable design. In one example, theepicyclic gearbox is a star gear system. Such an arrangement will bedescribed with reference to FIG. 3A. In another example, the epicyclicgearbox is a planetary gear system. Such an arrangement will bedescribed with reference to FIG. 3B.

The use of speed reduction device allows 111 allows the electric machineto operate at higher speed, and the fan to operate at lower speed. Inthis way, both components operate in their respective regimes of higherefficiency. Further, a larger fan is permitted due to the reduction oftip speed, and a smaller electric machine is permitted.

Thus, as illustrated in FIG. 2, the arrangement of FIG. 1 permits acombination of a fan having a high hub-tip ratio, and a high aspectratio electric machine, in terms of its length-to-diameter.

The hub-tip ratio of the fan 108 in the present example is the ratio ofthe diameter D_(F) of the leading edge of the fan blades 201, to thediameter of the diameter D_(H) of the hub 202 at the leading edge 203 ofthe fan blades 201, i.e. the diameter of the inner gas-washed surface ofthe fan 108. It will be appreciated that the hub-tip ratio is astandard, well-known property of a fan, compressor, or turbine stage.

The length of the electric machine as defined herein is the maximumlength L_(A) of the electromagnetically active components, whilst thediameter of the electric machine as defined herein is the maximumdiameter D_(A) of the electromagnetically active components. Examples ofthe definitions of length L_(A) and diameter D_(A) as applied tospecific machine types will be described further with reference to FIGS.4 and 5.

Benefits are obtained by specifying the following parameters for thepropulsor 101:

(i) the aspect ratio of electric machine 110, L_(A)/D_(A) (i.e. thevalue of L_(A) divided by D_(A)), being from 0.8 to 2;

(ii) the reduction ratio of the speed reduction device being at least3:1;

(iii) the ratio of the diameters of the electromagnetically activecomponents in the electric machine and the fan, D_(A)/D_(F) (i.e. thevalue of D_(A) divided by D_(F)), being from 0.3 to 0.5.

In particular, the inventor has discovered that this combination ofvalues advantageously enables the propulsor 101 to have a smallerdiameter nozzle 107 for the same overall fan pressure ratio, whichallows the pylon 103 to be made shorter. This results in, for aparticular fan diameter D_(F), greater ground clearance, or a greaterfan diameter D_(F) for a particular ground clearance. Further, thereduction in pylon length and height results in a reduced moment on thewing 102, and a reduction in weight.

In a specific embodiment, the ratio L_(A)/D_(A) is from 1.1 to 1.7. Theinventor has discovered that this allows a narrowed nozzle, along with ahigher speed machine. In another specific embodiment, L_(A)/D_(A) isfrom 1.3 to 1.5. In a more specific embodiment, L_(A)/D_(A) is 1.4.

In an additional or an alternative embodiment, the ratio D_(A)/D_(F) isfrom 0.35 to 0.45. In a specific embodiment, the ratio D_(A)/D_(F) isfrom 0.37 to 0.43. In a more specific embodiment, the ratio D_(A)/D_(F)may be 0.4.

The hub-tip ratio of the fan 108, i.e. the value of D_(T)/D_(H), may befrom 0.25 to 0.31. In a specific embodiment, the hub-tip ratio may befrom 0.27 to 0.29. In a more specific embodiment, the hub-tip ratio maybe 0.28.

In an embodiment, the fan 108 has a tip pressure ratio (i.e. the ratioof the stagnation pressure immediately upstream of the tip of a fanblade 201, and immediately downstream of the fan blade 201) of from 1.3to 1.7 at an altitude of 35,000 feet above sea level and a temperatureof minus 54 degrees Celsius (i.e. ISA+0 standard conditions), and a trueairspeed of Mach 0.85, i.e. during cruise conditions. In a specificembodiment, the tip pressure ratio may be from 1.4 to 1.6 in theaforesaid conditions. In a more specific embodiment, the tip pressureratio may be 1.5 in the aforesaid conditions. In another specificembodiment, the tip pressure ratio may be 1.38 in the aforesaidconditions.

A diagram of one embodiment of the propulsor 101 is shown in FIG. 3A, inwhich the speed reduction device 111 is a star-type epicyclic gearbox,indicated generally at 301. The electric machine 110 is coupled via aninput shaft to a sun gear 302, which meshes with planet gears 303mounted to a static carrier 304. The planet gears 303 transfer torque toa ring gear 305, which drives the fan 108. In an embodiment, thereduction ratio of the star gearbox 301 is from 3:1 to 3.7:1. Use of astar gearbox may assist in terms of reducing complexity of installationdue to the fixed carrier, for example with respect to lubricationsystems.

A diagram of another embodiment of the propulsor 101 is shown in FIG.3B, in which the speed reduction device 111 is a planetary-typeepicyclic gearbox, indicated generally at 311. The electric machine 110is coupled via an input shaft to a sun gear 312, which meshes withplanet gears 313 mounted in a rotating carrier 314. The planet gears 313transfer torque to a static ring gear 315, with the carrier driving thefan 108. In an embodiment, the reduction ratio of the planetary gearbox311 is from 3:1 to 4.7:1. Use of a planetary gearbox may assist in termsof providing a greater reduction ratio for a given number of gear teeth.

An example of a configuration of the electric machine 101 is shown incross-section through its central axis A-A in FIG. 4. This particularconfiguration is a radial flux electric machine 401, which comprises astator 402 which surrounds a rotor 403. The stator 402 comprises alamination stack of the known type. In the present example, the electricmachine is a permanent-magnet machine and thus the rotor 403 comprisespermanent magnets, which interact with the magnetic field generated bywindings in the stator 402 to generate torque. Alternative machine typessuch as induction machines may also be employed. As a radial fluxmachine, end windings 404 emerge axially from the lamination stack ofthe stator 402.

As described previously, the electric machine 401 may be described ashaving an electromagnetically active length L_(A) and anelectromagnetically active diameter D_(A). As used herein,“electromagnetically active” refers to the region responsible forgenerating torque upon the rotor. Thus in the present example, thelength L_(A) is the length of the lamination stack of the stator 402,and does not include the end windings 404 as they do not generate anappreciable torque upon the rotor 403. Similarly, the diameter D_(A) isthe diameter of the lamination stack of the stator 302.

Put another way, the “electromagnetically active” parts of the electricmachine 401 may be characterised as those components which form part ofthe torque-generating magnetic circuit in the machine.

Those skilled in the art will appreciate how this definition may readacross to other machine types such electromagnet-based synchronousmotors, and axial flux machines, with the definitions of aspect ratio asset out herein also applying to these types of machine.

Another example configuration of the electric machine 101 is shown incross-section through its central axis B-B in FIG. 5. This particularconfiguration is a radial flux electric machine 501, which comprises arotor 502 which surrounds a stator 503. As with electric machine 401,the electromagnetically active length L_(A) of this machine is thelength of the lamination stack of the stator 503, not including the endwindings 504. The electromagnetically active diameter D_(A) is, however,in this embodiment, the diameter of the rotor 502. Again, machine 501 isa permanent-magnet machine, and thus the diameter D_(A) is bounded bythe greatest radial extent of the magnets therein. Should the machineinstead be an induction machine, the diameter D_(A) would be defined bythe greatest radial extent of the rotor iron.

More generally, as described with reference to FIG. 4, it will beappreciated the “electromagnetically active” parts of the electricmachine 501 may be characterised as those components which form part ofthe torque-generating magnetic circuit in the machine.

Various examples have been described, each of which feature variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthe invention extends to and includes all combinations andsub-combinations of one or more features described herein.

1. An electric ducted fan for an aircraft, comprising: a nacelledefining a duct; a propulsive fan in the duct having a fan diameterD_(F); an electric machine configured to drive the fan, the electricmachine having an electromagnetically active length L_(A) and anelectromagnetically active diameter D_(A) defining an aspect ratio(L_(A)/D_(A)) of from 0.8 to 2; a speed reduction device between theelectric machine and the fan, having a reduction ratio of at least 3:1;and wherein a ratio of the electromagnetically active diameter D_(A) tothe fan diameter D_(F) (D_(A)/D_(F)) is from 0.3 to 0.5.
 2. The electricducted fan of claim 1, in which the aspect ratio (L_(A)/D_(A)/ is from1.1 to 1.7.
 3. The electric ducted fan of claim 2, in which the aspectratio (L_(A)/D_(A))/ is 1.4.
 4. The electric ducted fan of claim 1, inwhich the ratio of the electromagnetically active diameter D_(A) to thefan diameter D_(F) (D_(A)/D_(F))/ is from 0.35 to 0.45.
 5. The electricducted fan of claim 4, in which the ratio of the electromagneticallyactive diameter D_(A) to the fan diameter D_(F) (D_(A)/D_(F))/ is from0.37 to 0.43, for example 0.4.
 6. The electric ducted fan of claim 1, inwhich the electric machine is a radial flux electric machine.
 7. Theelectric ducted fan of claim 6, in which the stator is exterior to therotor, the electromagnetically active diameter D_(A) is the diameter ofa stator stack, and the electromagnetically active length L_(A) is thelength of the stator stack.
 8. The electric ducted fan of claim 6, inwhich the stator is interior to the rotor, the electromagneticallyactive diameter D_(A) is the diameter of the rotor, and theelectromagnetically active length L_(A) is the length of the statorstack.
 9. The electric ducted fan of claim 1, in which the fan has ahub-tip ratio of from 0.25 to 0.31.
 10. The electric ducted fan of claim9, in which the fan has a hub-tip ratio of from 0.27 to 0.29, forexample 0.28.
 11. The electric ducted fan of claim 1, in which thepropulsive fan has a tip pressure ratio of from 1.3 to 1.7 at analtitude of 35,000 feet and a true airspeed of Mach 0.85.
 12. Theelectric ducted fan of claim 11, in which the propulsive fan has a tippressure ratio of from 1.3 to 1.5 at an altitude of 35,000 feet and atrue airspeed of Mach 0.85.
 13. The electric ducted fan of claim 12, inwhich the propulsive fan has a tip pressure ratio of 1.38 at an altitudeof 35,000 feet and a true airspeed of Mach 0.85.
 14. The electric ductedfan of claim 1, in which the speed reduction device is a star gearboxwith a reduction ratio of at most 3.7:1.
 15. The electric ducted fan ofclaim 1, in which the speed reduction device is a planetary gearbox witha reduction ratio of at most 4.7:1.